This invention relates to electrical power assisted steering systems of the kind in which an electrical motor is adapted to apply an assistance torque to a steering component such as a steering column so as to reduce the driver effort required to control the vehicle.
In a simple electric power assisted steering system a torque sensor is provided which is arranged so that the level of torque in a steering column is measured. From this measurement a controller calculates the value of a torque demand signal that is indicative of the torque that is to be generated by an electric motor attached to the steering column. The motor applies a torque to the column of the same sense as that demanded by the driver and thus reduces the effort needed to turn the wheel.
In a typical apparatus, the relationship between the steering column torque and the assistance torque is set by a boost curve. This is a mapping between the column torque and the assistance torque where for a given input torque a multiplier value is defined that determines an amount by which the input signal is multiplied. To apply a boost, the value of the multiplier for a given input torque will be not unitary, 1.0. For example, if the multiplier is 2.0 then the output assistance torque will be twice the measured torque, if it is 0.5 it will be half the measured column torque. Instead of a multiplier it is also possible to define the relationship between input torque and assistance torque using a look up table, or in any other convenient manner. The boost curve will then be hard coded into the look up table.